Significant suppression of invasive emerald ash borer by introduced parasitoids: potential for North American ash recovery

Successful management of invasive forest pests with sustainable approaches, such as biological control, is critical to the restoration of the affected or damaged forest ecosystems. Several parasitoids introduced from Northeast Asia were released between 2015 and 2017 in several northeastern states of the USA for biocontrol of the invasive emerald ash borer (EAB), Agrilus planipennis. Using life tables to estimate the pest population growth rate, we evaluated the impact of two introduced parasitoids (Spathius galinae and Tetrastichus planipennisi) on EAB population dynamics in five ash-dominated hardwood forests in three Northeastern U.S states. We observed ~ 76% decrease in average densities of live EAB larvae to a low density (< 7 live larvae per m2 of tree phloem) from 2015 to 2020. This reduction in pest density was driven primarily by the significant increase in parasitism rates (from 35 to 78%) by S. galinae, along with low-to-moderate levels of mortality from local generalist natural enemies, such as woodpeckers. Spathius galinae alone caused a 31–57% reduction in the net pest population growth rate from 2018 to 2020. These findings demonstrate that in the recently invaded ash forests in the Northeastern USA, timely introduction of specialized natural enemies, such as S. galinae, along with local generalist natural enemies, may significantly suppress the invasive pest populations to low densities, allowing surviving trees to recover.